As an apparatus which may be used for measuring the critical dimension of specimens to a precision in the order of a nanometer, there is, for example, an electron microscope. An electron microscope forms an image of a specimen by enlarging it from several hundreds of times to several millions of times, and it measures the critical dimension of necessary parts. However, the critical dimension measured in the enlarged image is the number of picture elements or pixels contained therein. The critical dimension of a specimen is calculated by multiplying the number of pixels by the magnification of the image for the specimen. The question of whether the critical dimension of the specimen is a real value or not depends on whether the magnification is a real value or not.
As an apparatus that is capable of determining the precision of measurement of the critical dimension of specimens, there is a SEM (scanning electron microscope) for critical dimension measurement. The SEM for critical dimension measurement is now considered to be an apparatus that is indispensable for pattern dimension management in the fabrication process used in the manufacture of semiconductor devices. Because of the necessity for critical dimension measurements to correspond with real values in an SEM for critical dimension measurement, the magnification is calibrated with a pitch pattern having a known cycle of repetition. Patent Document 1 (Japanese Patent Laid Open Publication No. 2002-15691) describes a technique for measuring variations in magnification when the acceleration voltage and the working distance (distance between objective lens and specimen) have changed by using standard specimen, such as a mesh, and of calibrating the displayed magnification and a scale bar. Patent Document 2 (Japanese Patent Laid Open Publication No. 2000-337846) describes a technique for measuring variations in magnification resulting from an increase in the acceleration voltage, from the retarding voltage and from the specimen by using a pitch pattern having a known cycle of repetition and of calibrating the magnification by multiplying the sit magnification by a calibration coefficient.
The magnification range of an electron microscope extends from manufactures of several hundreds of times to several millions of times. However, the magnification range in which a calibration is possible by using a standard specimen is very limited. Specifically, the cycle of a pitch pattern realizable by the present processing art is several hundreds of nm. In order to average the processing errors, images containing 10 or more pitch patterns are taken. The cycle (number of pixels) of pitch patterns shown in the image is acquired from the Fourier transform image of the image, which calculates the magnification that turns out to be the cycle (nm) of the pitch patterns. The magnification at which an image contains approximately 10 pitch patterns is approximately 10,000 times. The patent publication mentioned above describes the calibration of magnification based on use of an SEM. However, electron microscopes include TEM (transmission electron microscope) and STEM (scanning transmission electron microscope) types in addition to the SEM type. The SEM visualizes the specimen structure by detecting an electron beam radiating from the specimen surface, while the TEM and STEM visualize the specimen structure by detecting an electron beam that has infiltrated into the specimen. For that purpose, the TEM and STEM slice the specimen into thin films for observation. Because of a limited dispersion of the electron beam within the specimen in the TEM and STEM, the space resolution of the TEM and STEM is higher by a digit than the resolution of the SEM. With the TEM and STEM, it is possible to take a lattice image, which is used together with the pitch pattern for the calibration of magnification. The lattice interval varies between the narrower limit of 0.102 nm for a gold single crystal and the wider limit of 1.0 nm for a mica single crystal. The magnification at which a lattice image can be observed is several millions of times or more. In other words, magnification calibration is possible by using specimens having a known cycle of repetition only when the magnification is 10,000 times or less or a million times or more.
Since a magnification cannot be calibrated by using a standard specimen between 10,000 times and a million times, a calculated value estimated from the control current of the electron optical system is typically used as a measure of the magnification of the image of the specimen. Specifically, in an apparatus like a SEM or STEM, wherein an image is formed by raster scanning a sharply focused electron beam on the specimen, the range of scanning or magnification of any freely chosen image is calculated on the basis of the current amplitude of the scanning deflector by assuming that the range of current of the scanning deflector and the scanning range of the incident electron beam are parallel. In an apparatus like a TEM wherein parallel beams are amplified by an electron lens, the magnification of any freely chosen image of the specimen is calculated by acquiring the optical magnification of the electron lens from the excitation current of the electron lens.
As described above, magnification is analyzed and calibrated from images actually taken only at very limited magnifications. At other magnifications, the magnification contains an error of approximately ±5% because of the use of magnifications calculated from the control current of the electron optical system. As nanotechnology continues to progress, a large number of industrial products formed in the nanometer scale are now being produced in addition to semiconductor devices. These products have a variety of dimensions, and the management of their dimensions is required at a variety of magnifications. As the device dimension decrease in semiconductor devices, it has become necessary to measure a pattern dimension at a magnification that is different from the magnification used for taking a pitch pattern.